Why can't reference to child Class object refer to the parent Class object?

I was explaining OOP to my friend. I was unable to answer this question.

I just escaped by saying, since OOP depicts the real world. In real world, parents can accommodate children but children cannot accommodate parents. same is the case in OOP.

class Parent
{
  int prop1;
  int prop2;
}

class Child : Parent // class Child extends Parent  (in case of Java Lang.)
{
  int prop3;
  int prop4;
  
  public static void Main()
  {
     Child aChild = new Child();
     Parent aParent = new Parent();
     aParent = aChild;// is perfectly valid.
     aChild = aParent;// is not valid. Why??
 
  }
}

Why isn't this statement valid?

 aChild = aParent;// is not valid. Why??

since aChild's members are superset of aParent's members. Then why can't aChild accommodate a parent.